John Cole passes along this concise WSJ account of the events leading to the present crash.
The 2001 recession might have ended the bubble, but the Federal Reserve decided to pursue an unusually expansionary monetary policy in order to counteract the downturn. When the Fed increased liquidity, money naturally flowed to the fastest expanding sector. Both the Clinton and Bush administrations aggressively pursued the goal of expanding homeownership, so credit standards eroded. Lenders and the investment banks that securitized mortgages used rising home prices to justify loans to buyers with limited assets and income. Rating agencies accepted the hypothesis of ever rising home values, gave large portions of each security issue an investment-grade rating, and investors gobbled them up.
…and everybody lived happily ever after.